The alkylation of benzene with acyclic olefins is a widely practiced commercial process. Alkylbenzenes are produced as a commodity product in large-scale facilities, e.g. often in amounts of 50,000 to 200,000 metric tons per year per plant. The alkylbenzenes find a variety of applications, the most prevalent of which is to be converted into detergents by sulfonation and neutralization. The alkylbenzene must meet stringent product specifications to be commercially acceptable. The benzene content of the product should be relatively free from benzenes, e.g., less than about 1 part per million by weight (ppmw), and often less than about 0.5 ppmw. Also, desirable alkylbenzene products are relatively free, e.g., less than about 50, preferably less than about 5, ppmw, from byproducts such as dialkylbenzenes, oligomers of olefins, and the like (herein referred to as “heavies”). The heavies content of a stream can be determined by ASTM D-2887-04a, Standard Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography, which is in effect on Jul. 31, 2004, available from ASTM International, West Conshohocken, Pa., United States of America.
Another purity issue associated with alkylbenzenes is the absence of color formers, or color bodies. Color bodies are components that impart color to the alkylbenzene. Saybolt color is one procedure for determining color of a liquid and for purposes herein refers to ASTM D-156-00, Standard Test Method for Saybolt Color of Petroleum Products (Saybolt Chronometer Method), which is in effect on Jul. 31, 2004, available from ASTM International. Desirable alkylbenzenes have a Saybolt color of at least +25, and preferably at least +29.
Often a bromine index is used to evaluate the quality of an alkylbenzene product. Although the bromine index relates primarily to olefin content, generally a correlation exists between the bromine index and the color of the alkylbenzene and of sulfonated alkylbenzenes made therefrom, e.g., as determined by the Klett color index. Typically, the bromine index of an alkylbenzene should be less than about 20, preferably less than about 5. There are a number of methods for determining a bromine index of an alkylbenzene, but the methods often provide results that are not consistent with each other. Hence, for purposes herein, the bromine index is that measured by UOP Method 304-90, “Bromine Number and Bromine Index of Hydrocarbons by Potentiometric Titration”, which is in effect on Jul. 31, 2004, available from ASTM International.
Typically, alkylbenzene is purified by the use of several distillation steps. For instance, see Pujado, Linear Alkylbenzene (LAB) Manufacture, in Handbook of Petroleum Refining Processes, by Robert A. Meyers, Second Edition, pp 1.53 to 1.66, McGraw-Hill, New York (1996), especially pages 1.56 to 1.60. In general, the alkylation reaction product is subjected to a first distillation in a benzene column to separate benzene as an overhead stream that can be recycled to the alkylation reaction. The bottoms stream from the benzene column is then subjected to a distillation to separate paraffins and unreacted olefin in a paraffins column. The paraffins-containing overhead is capable of being recycled to a paraffin dehydrogenation unit while the bottoms stream is passed to a heavy alkylate distillation column. In the heavy alkylate distillation column, heavies are separated from the lighter alkylbenzene, and a heavies-containing stream is withdrawn as a bottoms stream. If desired, the bottoms stream can be subjected to a further distillation to recover additional alkylbenzene.
Several modifications to this refining scheme have been proposed. For instance, in U.S. Pat. Nos. 4,423,278; 4,433,196 and 4,468,476, the use of clay treatment has been proposed to reduce color formers in the product. U.S. Pat. Nos. 4,795,550 and 6,031,144 disclose the use of acidic catalysts to reduce olefin content.
The stream to be acid treated could, for instance, be the feed to the heavy alkylate distillation column. Thus, the heavy alkylate distillation column could be used to remove heavies produced by the acidic catalyst. However, the amount of olefin-containing components contained in the feed stream is such that catalyst replacement or regeneration must be very frequent or very large volume catalyst beds must be used. Accordingly, where acid treatment has been used, it has been to treat the overhead from the heavy alkylate distillation column.
In one commercially practiced process, the alkylbenzene from the heavy alkylate distillation column is subjected to clay treatment and then is passed to a finishing distillation column. A benzene-containing overhead stream is withdrawn, and the desired alkylbenzene is recovered as the bottoms stream. This variation has the benefit of removing any benzene contained therein. Benzene, for instance, may be coproduced during the clay treatment, e.g., by transalkylation of the alkylbenzene to dialkylbenzene and benzene, or may result from degradation of the alkylbenzene during upstream distillation operations.
A number of factors arise in designing a distillation system for recovering alkylbenzene from the alkylation of benzene. One consideration is that components contained in the alkylation product, including the alkylbenzene, can undergo further reaction or degradation. For this reason, the temperatures in the distillation processes are maintained sufficiently low that undue reaction or degradation does not occur, usually below about 300° C., preferably below about 270° C. Thus, at least the heavy alkylate distillation column is operated at subatmospheric pressure, e.g., from about 0.5 to 10, often about 1 to 3, kPa absolute. Not only does operation at subatmospheric pressure entail energy expense to maintain the sought vacuum, but also the ability to increase the volume of alkylation product processed is impaired due to practical temperature limitations as well as the vacuum capacity. Additionally, it is generally desired to minimize the residence time of the alkylation product in the column at higher temperatures. Hence, lower reflux ratios are often preferred. These lower reflux ratios provide a risk that an excursion from the desired conditions in the column may occur and an instability would result. A further implication is that to maintain a desired overhead purity, that is, with little heavies in the alkylbenzene-rich overhead, the bottoms may contain significant amounts of the desired alkylbenzene product.
Other considerations, especially for commercial facilities, are energy and equipment integration and capital costs. A primary consideration for energy consumption of a refining system is the heat required for the reboilers for each of the distillation columns. A significant factor determining the reboiler heat, or duty, is the amount of reflux required to achieve the sought degree of separation, i.e., reflux ratio (R/F), which is the mole ratio of reflux to feed to the column. Thus, to maintain a given degree of separation, as the flow rate of feed is increased, the reboiler heat required must also be increased to enable the reflux ratio to be maintained.
Significant financial benefit can be achieved through even slight improvements in efficiency or reductions in energy consumption or increases in capacity in a given existing plant, e.g., through debottlenecking. Unfortunately, the demands placed upon alkylbenzene product quality give little room for debottlenecking the distillation train while maintaining advantageous operating costs.
Accordingly, processes are sought by which the product quality of the alkylbenzene product can be maintained or improved while increasing the capacity of a given distillation train for purifying alkylbenzene. Processes are also sought to reduce energy consumption for purification per unit of alkylbenzene product. Processes are additionally sought to obtain alkylbenzene having desirable absence of benzene and heavies where the catalytic treatment to remove olefins is subsequent to the separation of heavies.